Coordination of proliferation and neuronal differentiation by the retinoblastoma protein family.

Once neurons enter the post-mitotic G0 phase during central nervous system (CNS) development, they lose their proliferative potential. When neurons re-enter the cell cycle during pathological situations such as neurodegeneration, they undergo cell death after S phase progression. Thus, the regulatory networks that drive cell proliferation and maintain neuronal differentiation are highly coordinated. In this review, the coordination of cell cycle control and neuronal differentiation during development are discussed, focusing on regulation by the Rb family of tumor suppressors (including p107 and p130), and the Cip/Kip family of cyclin dependent kinase (Cdk) inhibitors. Based on recent findings suggesting roles for these families in regulating neurogenesis and neuronal differentiation, I propose that the Rb family is essential for daughter cells of neuronal progenitors to enter the post-mitotic G0 phase without affecting the initiation of neuronal differentiation in most cases, while the Cip/Kip family regulates the timing of neuronal progenitor cell cycle exit and the initiation of neuronal differentiation at least in the progenitor cells of the cerebral cortex and the retina. Rb's lack of involvement in regulating the initiation of neuronal differentiation may explain why Rb family-deficient retinoblastomas characteristically exhibit neuronal features.